France became the United States’ crucial ally after the American victory at Saratoga.
The Battles of Saratoga culminated in the surrender of British General John Burgoyne’s army on October 17, 1777. The victory demonstrated that the American cause could defeat a major British force and encouraged France to support the new nation openly.
France signed the Treaty of Alliance and the Treaty of Amity and Commerce with the United States in February 1778. French military aid, loans, troops, supplies, and naval forces became central to the American war effort. French participation helped produce the allied victory at Yorktown in 1781.
France had already provided secret assistance before Saratoga, including arms and supplies. Saratoga nevertheless changed the diplomatic situation by making a formal alliance more attractive. Spain also entered the war against Britain in 1779, but it did not sign the same formal alliance with the United States.